The invention relates to a circuit arrangement for the switchable amplification of variable electrical signals.
In the case of an ensemble of variable electrical signals, in particular of RF (radiofrequency) signals, with levels of different magnitudes, it is advantageous to match them at least in stages to approximately the same maximum level magnitude. Thus, by way of example, a downstream AD converter can always be driven optimally and the signal/noise ratio, called S/N ratio hereinafter, can be maximized. This generally requires an amplifier that is looped into the signal flow only as required, to be precise at correspondingly low levels. At correspondingly high signal levels, by contrast, the signal is passed through as far as possible without any attenuation, the original S/N ratio being maintained.
It is conceivable to permit a very low-noise amplifier to be permanently looped in and to insert a switchable attenuation element into the output path of the amplifier, which attenuation element attenuates the preceding gain again in the case of high levels. In this case, however, the S/N ratio deteriorates by the noise figure of the amplifier, which may be very small, however, with a value of 1 dB. The problem in this case, however, is that the amplifier must be configured in such a way that it can supply to the attenuation element an output level which exceeds the maximum possible input level by its gain. In order to obtain the signal precisely thus with little distortion, this requires a very high outlay on circuitry for the amplifier, which simultaneously requires a high DC power. A high DC power consequently leads to high heating of the amplifier, so that a waste heat problem additionally occurs in this case.
The opposite arrangement, connecting the switchable attenuation element upstream of the amplifier, is ruled out from the outset owing to the drastic deterioration of the S/N ratio.
A known standard configuration for solving the problem is an amplifier 28 with a switchable bypass 10 arranged outside the amplifier 28 in accordance with FIG. 1. In this case, the amplifier 28 shown is connected to the voltage source 9 via two supply terminals 15 and 16. In accordance with the represented switch position of the switching element 8, a supply voltage is thus present at the amplifier 28. By way of example, if the voltage source supplies a DC voltage of 5 V, then +5 V is applied to the first supply terminal 15, while 0 V is applied to the second supply terminal 16. The amplifier 28 is consequently activated. Furthermore, a circuit element S1 and S2 is respectively arranged between signal input 1 and amplifier input 3 and between amplifier output 4 and signal output 2. In this case, the two circuit elements S1 and S2 are electrically coupled to one another. In accordance with the represented switch position of the circuit element 8, +5 V, for example, is likewise applied to the control inputs 25, so that the two switching elements S1 and S2 are present in a manner coupled to one another in the switch positions represented. A signal fed in at the signal input 1 thus passes via the amplifier 28 to the signal output 2. In the case of the second switch position (not represented) of the switching element 8, no voltage is present at the two control inputs 24. The signal fed in at the signal input 1 consequently passes to signal output 2 in unamplified fashion via the switchable bypass 10. Since, in the case of this switch position, no supply voltage is present at the amplifier 28 either, the latter is deactivated.
One problem of this arrangement with the amplifier activated is the finite blocking attenuation of the switches brought about by the capacitive residual coupling. It should be at least as high as the gain. Otherwise there is the risk of self-excitation depending on amplifier type and phase rotation in the bypass branch. Owing to the capacitive nature of the residual coupling, the problem is aggravated toward high frequencies and, if appropriate, necessitates very complicated switch structures. Even if the blocking attenuation is not sufficiently higher than the gain, however, the finite negative feedback or feedback dependent on frequency or phase angle causes a frequency-dependent change in the gain.